I agree with Gordon, having the box does increase the potential resale value. Part of it for me is that if the seller has taken the trouble to keep the original box, they're more likely to have kept the set in good condition too, so I'm usually prepared to pay just that little bit more. Potential, but it really depends on who is looking for a particular set at a particular time.
Of course 95% of my boxes I discard after opening, but I'm not thinking of resale value but of storage practicality, I never buy with the thought of selling on, even in 20 years time. Now I have more storage space, I'll probably keep more boxes, especially for the big sets where I think it makes the most difference.